The present invention relates to small-sized microstrip antennas that operate on many different frequency bands. In particular, the invention relates to internal antennas used in mobile phones, which are fed from one feeding point.
In the present patent application, a frequency range comprises one or more frequency bands, i.e. a frequency band is part of the frequency range. Furthermore, by the reception band is meant a frequency band reserved for downlink data transmission and by the transmission band is meant a frequency band reserved for uplink data transmission.
In mobile stations, there is going on a changeover to terminals that operate in several frequency ranges. Solutions of several frequency ranges like this include so-called dual band terminals currently in use, which operate in two frequency ranges.
Dual band terminals have been implemented by both an external and internal antenna. The external antenna, which can be, for example, monopole, helix or their combination, is demanding as for its manufacturing technique, and it breaks easily. Therefore, in mobile stations, there is going on an increasing changeover to internal antenna structures implemented by microstrip antennas. The advantage of internal antennas compared to external antennas is the ease of the manufacturing technique and the speeding up of the serial production as the degree of integration increases, as well as the more durable structure than that of the external antennas.
A conventional microstrip antenna comprises a ground plane and a radiating antenna element that is insulated from the ground plane by an insulating layer. The resonance frequency of the microstrip antenna is determined on the basis of the physical dimensions of the antenna element and the distance between the antenna element and the ground plane. The operating principle and dimensioning of microstrip antennas are well known and they are described in the literature relating to the field.
FIGS. 1a and 1b show a microstrip antenna and an L-plane antenna according to prior art, which hereinafter in the present patent application will be called an L-antenna.
The microstrip antenna consists of a ground plane, a radiating antenna element, as well as a feeding line. In between and above the ground plane and the antenna element, there is either air or some other dielectric agent as an insulating material.
Traditionally, the L-antenna is a whip antenna that is bent near the ground plane parallel to the ground plane, whereupon the antenna has a low feed impedance. It is also possible to build of the L-antenna a microstrip antenna that consists of a ground plane, a radiating antenna element as well as a feeding line.
Normally, the length of the resonant proportion of the antenna in wavelengths is defined as the difference between the microstrip antenna and the L-antenna. The electric length of the microstrip antenna is half a wavelength whereas, traditionally, the electric length of the L-antenna is a quarter of a wavelength. From the electric length of the L-antenna it follows that the maximum current of the L-antenna is at the input.
Normally, the microstrip antenna is made on a double-sided substrate, one metallisation of which acts as the ground plane and on the other, the pattern of the antenna element is made by etching. The antenna element is fed by the feeding line, which is coupled to the antenna element either from one side (FIG. 1a) or by taking the feeding line through the ground plane and the insulating material (FIG. 1b). The resonance frequency of the microstrip and L-antennas is affected by the physical dimensions of the antenna element, the place of the feeding point, as well as, to some extent, the location of the antenna element with respect to the ground plane.
The size of the microstrip antenna has been reduced by developing a so-called PIFA antenna (PIFA, Planar Inverted F-Antenna), shown in FIG. 2b. In the PIFA antenna, the antenna element is coupled to the ground plane by a grounding line. This being the case, the actual antenna element can be dimensioned so that it is considerably smaller than in the case of the microstrip antenna. Furthermore, by optimising the place of the feeding point, the feed impedance of the antenna can be changed to the desired impedance level, which is not possible in the L-antenna. The resonance frequency of the PIFA antenna is affected by the physical dimensions of the antenna element and the ground plane, as well as by the distance of the antenna element from the ground plane. The antenna element is fed either from one side (FIG. 2a) or by taking the feeding line through the ground plane and the insulating material (FIG. 2b). When narrowing the width of the grounding line, the resonance frequency of the antenna decreases. The grounding line can be as wide as the whole antenna element or, at its narrowest, merely a conductor.
Furthermore, it is well known to feed a microstrip antenna capacitively. In a capacitively fed microstrip antenna, there is a feeding element in between the antenna element and the ground plane, whereupon a capacitive coupling is formed between the antenna element and the feeding element. The feeding line is coupled to the feeding element, which radiates power further to the antenna element. The capacitive coupling can be implemented both in the microstrip antenna (FIG. 3) and the PIFA antenna (FIG. 4).
The problem of microstrip antennas is the narrow bandwidth. The frequency ranges of 2nd generation mobile communication systems are reasonably narrow and, therefore, they can be implemented by microstrip antennas. For example, the frequency range of the GSM system is 890-960 MHz, wherein a transmission band is 890-915 MHz and a reception band is 935-960 MHz. Thus, the bandwidth required of one antenna element is no less than 70 MHz. Due to the production tolerances and the objects in the vicinity of the antenna, for example, the hand of a user, the bandwidth of the antenna element must be even wider. The frequency ranges required by 3rd generation mobile communication systems, for example, broadband CDMA systems are still considerably wider than, for example, the GSM system""s and, therefore, their implementation with microstrip antennas is difficult. For example, a transmission band of the WCDMA system is 1920-1980 MHz and a reception band is 2110-2170 MHz. This being the case, the whole width of the frequency range is 250 MHz. This is why the bandwidth of microstrip antennas according to prior art described above has been increased as far as possible with solutions, where several resonance frequencies close to each other are implemented in one antenna element.
Solutions are known from prior art, where several resonance frequencies close to each other are implemented in one antenna element. In one solution, the number of resonance frequencies is increased by adding slots to the antenna element. However, the slots easily act in the case of small antennas as slot radiators, whereupon antenna elements that are resonating close to each other are strongly coupled to each other and form a resonator around the slot. This further follows that at the frequency in question the radiation resistance is low and the current densities in the vicinity of the slot are high, whereupon the loss of the antenna increases. Consequently, the adding of the bandwidth of a microstrip antenna in the manner in question only succeeds at the cost of gain and radiation efficiency. Hence, with the solution in question, for example, the gain values required by 3rd generation broadband CDMA systems cannot be achieved.
Of the microstrip antennas described above, an attempt has also been made to develop antenna structures that operate in several frequency ranges. For example, an antenna structure of two frequency ranges can be implemented by one common feeding point and an antenna element the resonance frequency of which can be adjusted by a switch and an electric load to the frequency range of another mobile communication system. A second alternative is to use one antenna element and two separate feeding points, whereupon two different resonance frequencies are generated in the antenna element. A third alternative is to use two antenna elements, which are coupled to a common feeding point. In this case, both antenna elements have one resonance frequency.
FIG. 5 shows a PIFA antenna of two frequency ranges according to prior art, which is fed from one feeding point. The resonance frequency of the antenna element is adjusted either by coupling in between the antenna element and the ground plane an electric load. Alternatively, the load can also be coupled as part of the feeding line. The load can be some reactive component, for example, a capacitance or inductance. The size of the change in the resonance frequency is determined on the basis of the electric load.
A solution according to FIG. 5 is described, for example, in the publication xe2x80x9cElectrical Tuning of Integrated Mobile Phone Antennas,xe2x80x9d Louhos, J-P, Pankinaho, I, Proceedings of The 1999 Antenna Applications Symposium, Allerton Park, Monticello, Ill., Sep. 15-17, 1999. In the solution in question, it is possible to operate with one PIFA antenna element both on a transmission and reception band of the GSM900 system. The antenna element is dimensioned so that the first resonance frequency is selected from the reception band of the GSM900 system. The resonance frequency is adjusted to a lower resonance frequency by coupling the capacitive load C with a switch S between the antenna element and the ground plane, whereupon the resonance frequency of the antenna element changes to the transmission band of the GSM900 system.
FIGS. 6 and 7 describe the antenna structures described in the publication xe2x80x9cDual Frequency Planar Inverted F-Antennaxe2x80x9d (Liu Z., et al., IEEE Transactions on Antennas and Propagation, No. 10, October 1997, pages 1451-1458), wherein two resonance frequencies are implemented in one PIFA antenna.
In the solution according to FIG. 6, from a PIFA antenna E1, a part E2 is separated, which is dimensioned for a higher frequency range. The first antenna element E1 is fed from a feeding point F1 and the second antenna element E2 is fed from a second feeding point F2. Both antenna elements are grounded and dimensioned so that they have different resonance frequencies. For grounding, a plurality of ground pins G1, G2 are used. The antenna elements"" polarisations are the same.
In the solution according to FIG. 7, the antenna elements are coupled to each other, whereupon one antenna element E3 is formed, which is fed from one feeding point F3. For grounding, a plurality of ground pins G3, G4, G5 are used. In this case, in one slotted PIFA antenna, two resonance frequencies can be implemented. However, the dimensioning of the antenna elements becomes considerably more difficult, because the antenna elements are coupled to the same feeding point and the antenna elements"" gain, impedance and bandwidths depend on each other. Also in this solution, the antenna elements"" polarisations are the same.
The advantage of one feeding point compared to solutions of a plurality of feeding points is that the manufacturing of the antenna elements becomes easier and the need for contact surfaces decreases. The required area also becomes smaller. In addition, production, operators and the authorities want to measure the operation of an antenna, as well as the strength and quality of the signal transmitted and received by a mobile phone from one feeding point.
In the case of one feeding point and several antenna elements, the biggest problem is the inter-coupling of the antenna elements, which impairs the radiation efficiency of the antenna structure. Due to the inter-coupling of the antenna elements, from the antenna element that operates at a first frequency range, power is coupled to the antenna element of a second frequency range and vice versa. Therefore, in the solutions of several antenna elements in question, the harmful inter-coupling of antenna elements must be reduced in order to achieve good radiation efficiency.
In the solutions according to prior art described above, the antenna elements are parallel to the ground plane, whereupon the coupling between the antenna elements and the ground plane is highly capacitive. The capacitive coupling in turn follows that the antenna elements are unilateral. The transmitting antennas used in mobile stations should be unilateral, whereas their receiving antennas should be as isotropic, i.e. omnidirectional as possible. For example, the antenna structure according to FIG. 5 operates well when information is transmitted from a mobile station to a base transceiver station, but information transmitted by the base transceiver station should be received in all the different operating positions of the phone.
Although, in the solutions mentioned above, it is possible to change from one frequency range into another, the solutions are implemented in the GSM system, i.e. with reasonably narrow bandwidths. In addition, the antenna elements are unilateral, whereupon they do not necessarily operate sufficiently well when receiving a broadband signal. On the other hand, the problem with the antenna structure of two antenna elements fed from one feeding point is, in addition to those mentioned above, also the inter-coupling of the antenna elements. Hence, it has not been possible to implement antenna solutions required by 3rd generation mobile stations that meet the gain, radiation efficiency and bandwidth values, by microstrip antennas according to prior art.
Due to the factors mentioned above, by microstrip antennas according to prior art, it has neither been possible to implement an antenna structure comprising one feeding point that would operate optimally enough in both 2nd and 3rd generation mobile stations.
In the present invention, an antenna structure fed from one feeding point that operates on several different frequency bands with which in addition to a good bandwidth also unilaterality in transmitting and isotropy in receiving is achieved, is implemented in a new way. The antenna structure""s gain and radiation efficiency are made good by reducing the interfering inter-coupling of the antenna elements. In addition, due to the positioning of the antenna elements, the space required by the whole antenna structure is smaller compared to the antennas of a corresponding frequency range. Consequently, it is easy to position an antenna structure according to the invention, for example, inside a mobile phone or an antenna unit to be coupled to a mobile phone.
The objectives of the invention are achieved by both a new frequency band solution and a new positioning of antenna elements, which enables the implementation of an antenna structure that operates on a broad band. In the frequency band solution, the antenna""s transmitting antenna element of a lower frequency range is more unilateral than the receiving antenna element of a higher frequency range. In addition, the positioning of antenna elements according to the invention reduces the inter-coupling between at least two antenna elements, whereupon the antenna structure""s gain and radiation efficiency become good.
The basic idea of the invention is to use, instead on one transmitting and receiving antenna element, two antenna elements coupled to each other with a coupling line so that a first antenna element is used to receive information from a reception band of a first radio system and a second antenna element is used to transmit information on a transmission band of the first radio system. In a preferred embodiment of the invention, the first reception band is a reception band of some broadband CDMA system of a 3rd mobile station generation and the first transmission band is a transmission band of the same broadband CDMA system. In this way, the antenna structure is made to operate on a broad band and it is possible to operate in a broad frequency range.
According to the invention, the antenna elements are positioned so that the first antenna element, which preferably is a receiving antenna element, is on the side of the ground plane and perpendicular to the ground plane and the second antenna element, which preferably is a transmitting antenna element, is in turn above the ground plane and parallel to the ground plane. This being the case, the first antenna element can be made omnidirectional and the second antenna element unilateral. There is also little harmful inter-coupling between the antenna elements, whereupon a good gain and radiation efficiency are achieved by the antenna structure.
Harmful inter-coupling can be further reduced by designing the polarisations of the first and second antenna elements to differ from each other, whereupon a good polarisation attenuation is produced between the antenna elements.
By improving the coupling between the resonances of the first antenna and the ground plane, the efficiency and omnidirectionality of the antenna can be improved on the reception band. This can be best implemented so that the open end of the first antenna element is located in the vicinity of the upper edge of the printed board, whereupon the electric fields of the antenna and the ground plane are strongly coupled to each other at the xe2x80x9copenxe2x80x9d end of both radiators. This being the case, the antenna element acts as a feeding element for the ground plane, which acts as a main radiator.
The coupling between the second antenna element and the ground plane can again be reduced by placing the second antenna element on the ground plane so that the open end, feeding point and ground point of the second antenna element are located more in the centre of the ground plane. In this case, according to a preferred embodiment, the antenna structure can be placed in a mobile station that has, for example, a camera and a GPS antenna.
In the preferred solution, the adaptation of the first antenna element can be improved further by designing a coupling line connecting the antenna elements from the input to the second antenna element and a grounding line reaching from the second antenna element to the ground so that their common electric length is a quarter of a wavelength at the resonance frequency of the first antenna. This being the case, the first antenna element sees the grounding in question as open and the antenna operates more efficiently as a monopole-type (e.g. folded monopole) antenna. This also follows that although the grounding line of the first antenna element is slightly shorter than a quarter of a wavelength, its effect is smaller on the adaptation of the first antenna element than on the adaptation of the second antenna element and, thus, the capacitance of the first antenna element with respect to the ground plane is lower in the optimum location of the first antenna element so that radiation resistance and feed impedance of the first antenna element are sufficiently high.
The suitability of the antenna solution according to the invention for end products can be further improved with a preferred embodiment according to the invention, wherein the second antenna element is arranged to also operate in the frequency range or part of the frequency range of a second mobile communication system. In this case, for example, an antenna structure can be implemented, wherein by the first antenna element a reception band of a broadband radio system is implemented. By the second antenna element, both a transmission band of a broadband radio system and at least one transmission band of a second radio system, which is e.g. a transmission band, a reception band or both of the GSM1800 or GSMA1900 system, are implemented.
There always remains a little harmful, lossy inter-coupling between the antenna elements, which makes it more difficult to implement the second antenna element as adjustable. In the case in question, however, the implementation of the second antenna element becomes easier due to the first antenna element, because the first antenna element improves slightly the adaptation of the second antenna element at a lower resonance frequency on said frequency band of the GSM1800 or GSMA1900 systems and, thus, simultaneously adds to said bandwidth. Consequently, by the antenna structure according the invention, it is possible to implement an antenna structure that operates both in 2nd and 3rd generation mobile communication systems.
In the antenna structure according to the invention, the antenna elements do not significantly impair each other""s properties, whereupon it is easy to add to the same feeding point antenna elements that operate below and above the first transmission band. Thus, the operation of the antenna structure according to the invention can be extended, for example, into the frequency ranges of the GSM900 or PDC800 systems by using antenna elements dimensioned for the frequency ranges in question. The adding of antenna elements that operate above the first frequency range is even easier, because as the frequencies increase, the size of the antenna elements becomes smaller. It is easy to implement in the antenna structure, for example, at least one of the antenna elements of the following systems: Bluetooth, WLAN (Wireless Local Area Network) or GPS (Global Positioning System).
According to a first aspect of the invention, there is implemented an antenna structure, which comprises a first antenna element, a second antenna element, a ground plane for grounding the antenna structure, a coupling line for coupling the first antenna element and the second antenna element to each other, and a feeding line for feeding the antenna structure through one feeding point, in which antenna element (=antenna structure!), the first antenna element is next to the ground plane and perpendicular to the ground plane and the second antenna element is above the ground plane and parallel to the ground plane.
According to a second aspect of the invention, there is implemented a method for coupling a signal to an antenna structure, which comprises a first antenna element, a second antenna element, a ground plane for grounding the antenna structure, a coupling line for coupling the first antenna element and the second antenna element to each other, a feeding line for feeding the antenna structure, and which method comprises coupling signals to be transmitted and received to the antenna structure through one feeding point, the method comprising positioning the first antenna element next to the ground plane and perpendicular to the ground plane and positioning the second antenna element above the ground plane parallel to the ground plane.
According to a third aspect of the invention, there is implemented an antenna unit, which comprises an antenna structure, which antenna structure comprises a first antenna element, a second antenna element, a ground plane for grounding the antenna structure, a coupling line for coupling the first antenna element and the second antenna element to each other, and a feeding line for feeding the antenna structure through one feeding point, and which antenna structure is manufactured on an insulating material which has a base and at least one wall region, which wall region reaches in a direction deviating from the base, and the shape of which antenna structure follows the shapes of the base and the wall region, and in which antenna structure the first antenna element is next to the ground plane and perpendicular to the ground plane and the second antenna element is above the ground plane and parallel to the ground plane.
According to fourth aspect of the invention, there is implemented a mobile station, which comprises an antenna structure, which antenna structure comprises a first antenna element, a second antenna element, a ground plane for grounding the antenna structure, a coupling line for coupling the first antenna element and the second antenna element to each other, and a feeding line for feeding the antenna structure through one feeding point, and in which antenna structure the first antenna element is next to the ground plane and perpendicular to the ground plane and the second antenna element is above the ground plane and parallel to the ground plane.